Sobes.tech
Interview Help
Resources
Interview Preparation
Resources
Job Search
Resources
Legal
Back to questions
Junior

How to view the list of file descriptors in Linux?

sobes.tech AI

Answer from AI

The list of file descriptors can be viewed using the lsof command.

# Display all open files and descriptors for all processes
lsof

# Display open files and descriptors for a specific process by PID
# Replace <PID> with the process ID of interest
lsof -p <PID>

# Display open files and descriptors for all processes of a user
# Replace <username> with the username
lsof -u <username>

# Display open files and descriptors for a specific command
# Replace <command> with the command name
lsof -c <command>

# Display open files and descriptors related to network connections
lsof -i

# Display open files and descriptors related to a specific port
# Replace <port_number> with the port number
lsof -i :<port_number>

# Display open files and descriptors related to a specific file or directory
# Replace <file_or_directory> with the path to the file or directory
lsof <file_or_directory>

lsof outputs information in columns:

  • COMMAND: The name of the command associated with the process.
  • PID: Process ID.
  • USER: The user owning the process.
  • FD: File descriptor. Can be:
    • cwd: Current working directory.
    • rtd: Root directory.
    • txt: Text file (executable code).
    • mem: Memory map file.
    • mmap: Mapped file in memory.
    • <N>u: File descriptor <N> opened in "read/write" mode.
    • <N>r: File descriptor <N> opened in "read only" mode.
    • <N>w: File descriptor <N> opened in "write only" mode.
    • <N>l: File descriptor <N> is locked.
  • TYPE: Node type:
    • REG: Regular file.
    • DIR: Directory.
    • CHR: Character device.
    • BLK: Block device.
    • FIFO: Named pipe.
    • SOCK: Socket.
  • DEVICE: Device number.
  • SIZE/OFF: File size or offset.
  • NODE: Inode number.
  • NAME: File name.

You can also use the /proc filesystem. For each process (PID), /proc/<PID>/fd/ is a directory containing symbolic links to the files opened by that process. Each link is named after the file descriptor number.

# View file descriptors for a process with PID <PID>
# Replace <PID> with the process ID of interest
ls -l /proc/<PID>/fd/

The output of ls -l will show the descriptor number and the file it points to.

PreviousNext